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468

Jib Crane.

gives stresses due to bridge-weight, and 427 those due to loco-
motive, &c. Tabulate the stresses so as to find the maxima,
thus:

Member.
	ROLLING LOAD.
					Maximum live load.
		Stress due to dead load.
	Total maximum.

	ISt
 position
	2nd position
	3rd position
	4th position
	. 5th position
	H-
	-
		
AH HG BA &c.
	
	
	
	
	
	
	
	
	4. -f

	
	
	
	
	
	
	
	
	
	
	
	
	
	
	
	
	
	
	
	
	
	
	
	
	
	
	
After which the bars are designed to meet the stresses, either as
ties or struts. A lattice girder is shewn in Fig. 426, being two
Warren girders superposed.

Example 37.—The post, tie rod, and jib of a crane (Fig. 428) are
15, 45, and 50 feet long respectively. Find all the stresses, (i) with
barrel on tie rod ; (2) with barrel on jib, with a load of 5 tons. (Eng.
Exam., 1887.)

w

Reactions.—Load produces both turning effect and downward pull,
resisted by equal horizontal forces at b and c, and by 5 tons upward
force at c. That at b is supplied partly by bending strength of post,
and partly by balance-weight. Letter the truss.

First, suppose the weight hung from a as a fixed point. Draw
B C in stress diagram = 5 tons, and complete triangle B C A. Passing
to b} E A must be a pull to product equilibrium, and B AE is the force
triangle. Finally, forces at c are shewn by polygon A C D E A, and
stresses are :

onAB=i5-      on CA = i6*66-h      and on A E = 277 -